On vehicle fuel supply systems, the actual supply member (carburetor and petrol or injection pump) is connected to the fuel tank by systems consisting of two or three side by side conduits, which provide for fuel supply and feedback to and from the supply member and (the third conduit, which may be dispensed with) for recirculating the fuel vapours. For transport and storage reasons, these conduits, which are both long and complex in shape, are formed by butt connecting a number of pipes. For example, each conduit may comprise at least a long straight center pipe running underneath the vehicle floor, and two curved end pipes of given shape, for respectively connecting the straight pipe to the tank and the supply member. The connection of these pipes, which is currently done at the on-vehicle assembly stage, poses various problems. On the one hand, it complicates the assembly process, while, on the other other, and more importantly, it may result in trouble due to the thermal expansion the center pipe in particular is subject to in service. This, in fact, as it expands against the joints, sags and, if not adequately supported, may even scrape against uneven road surfaces, thus resulting in damage and fuel leakage. The same also applies, of course, in the case of failure of the joints, should the pipe be restrained too rigidly.